1. Field of the Invention
The present invention is related to a process of manufacturing a printed circuit board with plated landless through-holes by the use of a filling material, a radiation curable putty material, to protect the through-holes during the process.
2. The Prior Art
Japanese Early Patent Examination (KOKAI) 62-202587 discloses a process of manufacturing printed circuit board with electrically conductive landless through-holes by the use of a radiation curable putty material. This process, so called a filling material process, includes a) drilling through-holes in a substrate; b) plating interiors of the through-holes and the substrate surfaces; c) filling the putty material in the through-holes and coating the material on the substrate surfaces; d) exposing the material under radiation of light in order to cure the material; e) removing the cured coated material only; f) placing a dry film resist on the surfaces; g) exposing the resist layer using a photo-tool art work; h) developing the film; h) etching the substrate surfaces; i) removing the cured putty material in the through-holes and the remaining portion of the film. However, since minute empty space is often created in between the film and the end of the cured putty material, an etching solution invades into the space. As a result, this causes damaged edges of the through-holes after the completion of the process, as shown in FIG. 11.
Japanese Early Patent Examinations (KOKAI) 61-139089 and 58-100493 disclose the filling material process employing a photo imageable liquid resist for etching in the coating step. It seems that the employment of the liquid resist eliminated the empty space between the resist and the end of the cured putty material. Nevertheless, cure depth of the filling material used in the process is insufficient to protect the through-holes in the entire process, and the volume contraction of the material still causes the invasion of the etching solution between the resist and the end of the cured putty material in the through-holes.